In hospitals, public restrooms and other facilities, it is desirable to provide a lavatory faucet which turns off automatically after a given period of time, so that the water will not run indefinitely if the user does not shut off the faucet. Conventional faucets for this type of operation, typically are referred to as "metering faucets" and are mechanical in nature being operated by a piston. When the piston is moved by the user to open the valve, a variable chamber within the valve expands and fills with air or water. Upon being released by the user, the piston moves due to spring force and decreases the size of the chamber at a rate which is governed by the release of the air or water through a metering orifice. This restricts the rate at which the spring force moves the piston to close the valve and thus the period that water flows from the faucet. Among the draw backs with such a mechanical faucet is that the mechanical parts wear with repeated use and the metering orifice clogs, eventually requiring either replacement or periodic adjustment and cleaning to ensure the valve remains open for a satisfactory period of time for the user to wash.
As an alternative to these mechanical automatic faucets, electrically controlled solenoid valve models have been proposed. However, a conventional solenoid valve remains open only while an electric current is applied to an electromagnetic coil of the solenoid actuator. Upon removal of the electric current, a spring forces the solenoid to close the valve. As a result solenoid valves use a considerable amount of electricity which makes them impractical for battery powered operation. Therefore, power typically is supplied from the electrical wiring of the building, thus requiring an electrical outlet underneath the lavatory and protection against the user receiving an electrical shock in the event of a circuit failure.